1. Field of the Invention
The present invention relates to light emitting elements employing optical microcavities. Such an element may be used, for example, as a display element, a light-emitting device for data communication, a read/write head for data files, or a printing device and the like in the field of delta communication.
2. Description of the Prior Art
In known light-emitting elements employing organic material only one emission spectrum, which depends: upon the nature of the fluorescent material, can be obtained when the light emitted from the organic thin fluorescent film is used directly. It is possible to provide a color filter in front of the light emitting element, to remove part of the emission spectrum. However, the peak intensity of the light obtained is then smaller than the intensity of emission spectrum when no color filter is used, so that the efficiency is greatly decreased. There has been proposed, for instance, in JP-A-57-51781 an electroluminescent cell in which an organic light-emitting material and an electrically insulating binder are interposed between transparent substrates having transparent electrodes such as indium tin oxide. This device does not employ the micro-cavity principle described below.
There has further been proposed a multi-color light-emitting electroluminescent element which has a plurality of inorganic light-emitting layers of different emission colors that are formed on the same substrate by utilizing different dopants in zinc sulfide (JP-B-5-15073). This element also does not use the micro-cavity phenomenon.
The concept of optical micro-cavities, which is employed in the present invention, is known. In an optical micro-cavity, the optical length is sufficiently small that it determines the peak wavelength of the emitted light. Attempts have been made to develop such devices for both spontaneous emission and stimulated emission (as in a laser device). Such a device has been realized using the inorganic semiconductor GaAs, but is expensive to make and is inapplicable to the production of a multi-color light-emitting device, because of the narrow emission frequency range of GaAs.
Applied Physics Letters 63 (5), PAGES 594-5, T Nakayama et al. "Organic-photo and electro-luminescent devices with double mirrors", an article published August 1993, describes an earlier development in this field by one of the present inventors and others which also forms the subject matter of PCT/JP93/01342 (expected to be published in March 1994). Reference should be made to this published article, which describes optical micro-cavity effects in devices containing organic thin films, for background information in relation to the present invention.